It is generally known to coat substrates with a diamond-like carbon coating (DLC). DLC coatings are amorphous hydrogenated carbon films (a-C:H) characterised by a high hardness, a low coefficient of friction and excellent wear resistance.
However, because of the high compressive residual stresses in the coating, the adhesion of DLC coatings to the substrate is often too poor. This is an important drawback of DLC coatings, limiting the use of DLC coatings for certain applications.
Many attempts have been made to improve the adhesion of a DLC coating to the substrate for example by using an intermediate layer between the substrate and the DLC coating. An example of such an attempt comprises the use of a diamond-like nanocomposite (DLN) layer as intermediate layer between the substrate and the DLC layer as described in WO 98/33948.
Diamond-like nanocomposite coatings comprise C, H, O and Si. Generally, DLN coatings comprise two interpenetrating networks a-C:H and a-S:O. DLN coatings are commercially known as DYLYN® coatings (Diamond-Like Nanocomposite Coatings).
Also the use of a titanium or a titanium-based intermediate layer between the substrate and a DLC layer is known in the art to improve the adhesion of a DLC coating to a substrate.
For this type of layered structures a gradual change from a titanium or titanium-based layer to a DLC layer is required.
Therefore, first a titanium or a titanium-based layer is deposited on a substrate by means of a PVD process, subsequently DLC is deposited on the titanium or titanium-based layer. The composition of the titanium or titanium-based layer is thereby gradually changed from a titanium or titanium-based layer to a DLC layer.
However, a serious problem of the use of titanium or titanium-based interlayers is the target poisoning of the titanium target. Because of this target poisoning, the transition between the titanium or titanium-based layer and the DLC layer is not reproducible.
Another disadvantage of the application of a layered structure of a titanium or titanium-based layer and a DLC coating is the poor corrosion protection that is obtained.
This is the consequence of the bad surface coverage (line-of-sight locations) on for example the backside surfaces of a substrate to be coated. These surfaces are not exposed to the beam of titanium atoms.